Formable acid resistant titanium alloys



Unite FORMABLE ACID RESISTANT TITANIUM ALLOYS No Drawing. ApplicationNovember 15, 1956 Serial No. 622,275

6 Claims. (Cl. 75175.5)

This invention relates to titanium alloys, and more particularly toalloys containing titanium as the major constituent, which have goodworking properties and are highly resistant to corrosion.

While titanium metal is resistant to corrosion by most chemical-s, it israpidly attacked by fluoride salts and strong acids of the reducingtype, including hydrochloric, hydrofluoric, sulfuric, phosphoric,oxalic, formic and tn'chloro acetic acids. For example, in boiling 20%hydrofluoric acid, the titanium corrodes at a rate well in excess of1000 mils per year. At moderate temperatures, titanium resists corrosionby these acids only when they are very dilute, or when inhibitors of theoxidizing type are present in the acid or on the surface of the metal.The titanium alloys are generally more resistant than the unalloyedmetal; however, in all but a few isolated instances the corrosion rateis prohibitive. There are many applications, particularly in connectionwith hot hydrochloric and hot sulfuric acid, in which titanium alloysand also other commercially available materials do not have satisfactorycorrosion resistance. There has been a long felt need for a reasonablypriced corrosion resistant metal which has good working properties forservice in connection with the aforementioned concentrated acids.

The only known titanium alloys which are resistant to these strong acidsolutions are described in U. S. Patent No. 2,614,041 of Walter L.Finlay. They are binary alloys, containing between 30 and 40%molybdenum, and corrode at a rate up to about 10 mils per year inboiling 20% HCl or 40% H 80 However, these Ti-(30- 40% )Mo alloys tendto be quite brittle, especially toward the upper end of the specifiedmolybdenum range, and as a result, are difiicult to fabricate.

'In accordance with the present invention, it has been found thattitanium alloys containing vanadium or columbium or both in addition tomolybdenum, are at least as resistant, and in most instances moreresistant, to corrosion by the above-mentioned acids than the alloysdisclosed in U. S. Patent No. 2,614,041. Furthermore, the hardness ofthe novel alloys is such that they are more easily worked than thealloys of the patent. More particularly, the present inventioncontemplates formable, corrosion-resistant alloys containing about 20 to30% molybdenum, and about 1 to 25% vanadium or about 1 to 30% columbium,or up to 30% in total amount of vanadium and columbium, and the balancetitanium.

In the present alloys, substantial amounts of titanium and molybdenum ofthe alloys of the Finlay patent are replaced by vanadium and/orcolumbium. It was found that large additions of vanadium or columbium totitanium metal alone did not appreciably improve the resistance oftitanium to corrosion by boiling sulfuric or hydrochloric acids. Thepresence of molybdenum in substantial amounts is definitely necessary toimpart corrosion resistance, although the molybdenum content of thepresent alloys is less than that of the binary alloys Sttes Patent2,819,960 Patented Jan. 14, 1958 disclosedin the aforementioned patent.As an illustration, appreciable reductions in molybdenum content arepossible when the vanadium or columbium additions are more thanequivalent to the molybdenum replaced from a Ti-30% Mo alloy, forexample.

While columbium and vanadium are thus partial 'sub stitutes formolybdenum, it has been found that the desirable very low rate ofcorrosion is dependent upon the presence of at least 20% molybdenum inthe novel ternary or quaternary alloys. In the case oftitaniummolybdenum-vanadium alloys, the molybdenum content is preferablybetween about 24 and 30%, since those containing less than about 24%molybdenum are slightly more difiicult to work. Generally speaking,titaniummolybdenum-vanadium alloys in which molybdenumis below thepreferred range, while entirely satisfactory from the standpoint ofcorrosion resistance, are the least easily formed of any of the alloysof the present invenvention. No such preferredrange is apparent in thetitanium-molybdenum-columbium alloys, and the ease with which they areformed is substantially the same over the 20 to 30% molybdenum range.The same is true of the present quaternary alloys.

In the ternary vanadium alloys, the content of this element is betweenabout 1 and 25% byweight, with 5 to 20% being preferred. The ternariescontaining columbium have a somewhat higher tolerance for that elementthan the corresponding vanadium-containing alloys, andcolumbium may bepresent from 1 to 30%. Vanadium and columbium presumably function insubstantially the same manner to impart improved workability andcorrosion resistance, and when both are present together, they mayaccount for up to about 30% by weight of the quaternary alloy.

Several of the alloys of the present invention were given severecorrosion tests, along with samples of both Ti-30% Mo and Ti-40% Mo, anda Ti-25% Mo alloy. Several samples of each alloy were exposed forperiods of 48 hours to boiling 40% sulfuric acid and to boiling 20%hydrochloric acid. The results are conveniently stated in terms of milsper year of corrosion as determined by weight loss of the specimens.Corrosion rate as well as Rockwell A hardness and rollability of each ofthe alloys is reported in the following table, with the results forTi-30% Mo and Ti-40% Mo being the average of several tests.

Corrosion Rate in mils/year after 48 Composition Percent, Hardness,hours in boiling Balance Titanium Rockwell A Rollability 61 good 34 39 16. 3 Z 9. 1 2 2. 9 2 2. 5 2.8 4. l 3. 3 3. 5 2.0 1.6 1. 2 5. 3 1.0 4. 33. 4 2. 3 2. 5 3. d 2. 3 3. 6 24Mo-20V 3. 9 3. 1 22.5Mo-25V 4. 8 6. 7 2M030V 8. 5 12. 9

1 Range of several samples. 2 Average of several samples.

The pattern in the titanium-molybdenum binary alloys is readilyapparent. With increased molybdenum, corrosion resistance is improvedbut at the expense of work ability of the alloy. The ternary alloys ofthe present invention, on the other hand, exhibit not only goodworkability, but also very high resistance to corrosion over theaforementioned molybdenum, columbium and vanadium ranges. The optimumcolumbium ternary of those reported from both standpoints, containsabout 20% columbium and from 20 to 25% molybdenumand the balancetitanium. The corresponding optimum vanadium alloy is more sharplydefined at about 25% molybdenum and about 15% vanadium.

It will be seen from the above table that alloys were chosen havingmolybdenum contents near the upper end of the specified range, withcolumbium and vanadium 7 near the lower ends of their respective ranges,and vice versa, which permitted a fairly narrow range of titaniumcontent. However, this inverse relationship between molybdenum contentand that of columbium and vanadium, while it may be preferred, is notnecessary since the latter two elements are also substitutes fortitanium.

Numerous alloys outside the above-specified ranges were tested,including those containing 0-30% molybdenum and up to 50% of columbiumand vanadium each alone and in combination. However, those having amolybdenum content below about 20% with columbium and vanadium aboveabout 30% were found to be substantially harder than those of thepresent invention, and were sufliciently diflicult to work by presentlyknown methods to preclude any commercial possibilities.

Since tantalum is a natural impurity in columbium, the possibility thatthis metal itself might produce the aforementioned desirableimprovements in titaniummolybdenum alloys was considered. However, atitanium-40% tantalum alloy was found to corrode at a rate substantiallyof the order of unalloyed titanium. The corrosion rates, hardness, androllability of Ti-30%Mo- 2%Ta, Ti-25%Mo-5%Ta, and Ti-20%Mo-l0%Ta werefound to be approximately the same as those of corresponding Ti-Moalloys without tantalum additions. It is thus apparent that tantalum byitself does not benefit or harm titanium-molybdenum alloys, but acts asan inert element.

Likewise, it was found that tantalum is similarly inert in the alloys ofthe present invention. Thus, a few percent of tantalum may permissiblybe present therein along with columbium, and the appended claims are tobe so interpreted.

The novel, formable, corrosion-resistant alloys of the present inventionare well suited for use in connection with corrosive salts and strongacids of the reducing type, and particularly as materials ofconstruction for containing these corrosive chemicals. The alloys areeasily formed into any desired shape as by rolling or stamping or byother conventional working methods.

Titanium as employed in the foregoing description 'and in the appendedclaims is intended to include not only highly pure titanium as preparedby the well-known iodide method, but also commercial titanium, which maycontain one or more of the elements oxygen, nitrogen and carbon inamounts up to about 0.2% nitrogen, up to about 0.3% oxygen and up to0.5% carbon.

What is claimed is:

l. A formable titanium alloy, characterized by a high resistance tocorrosion by acids of the reducing type, containing about 20 to 30%molybdenum, at least one element selected from the group consisting ofabout 1 to 25% vanadium and about 1 to 30% columbium, the. total amountof vanadium and columbium not exceeding 30%, and the balancesubstantially titanium.

2. A formable corrosion-resistant alloy containing about 20 to 30%molybdenum, about 1 to 25% vanadi- References Cited in the file of thispatent UNITED STATES PATENTS 2,754,203 Vordahl July 10, 1956

1. A FORMABLE TITANIUM ALLOY, CHARACTERIZED BY A HIGH RESISTANCE TOCORROSION BY ACIDS OF THE REDUCING TYPE, CONTAINING ABOUT 20 TO 30%MOLYBDENUM, AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OFABOUT 1 TO 25% VANADIUM AND ABOUT 1 TO 30% COLUMBIUM, THE TOTAL AMOUNTOF VANADIUM AND COLUMBIUM NOT EXCEEDING 30%, AND THE BALANCESUBSTANTIALLY TITANIUM.